1. Field of the Invention
The present invention relates generally to maneuver performance of an aircraft and, more particularly, to a system for offsetting the aeroelastic effect when performing roll maneuvers without increasing the structural rigidity of the wing. In another manner of speaking, the concept of the invention is to avoid adverse wing twist and increase the aerodynamic effectiveness of the control surfaces of high performance tactical aircraft.
2. Description of the Prior Art
Whenever trailing edge control surfaces are employed to perform roll maneuvers, the wing twists, rotating the leading edge downward, resulting in reduced lift and consequently decreased rolling moment. This is called the aeroelastic effect. With increasing dynamic pressure or speed the aileron effectiveness (i.e. its power to produce rolling moment) gradually reduces to zero. This velocity at which the roll rate becomes zero (see FIG. 1, curve TE-CS) is called the roll reversal speed. To achieve a required roll rate within the flight regime, the torsional stiffness of the wing box must be increased. This calls for increased structural weight and consequently decreased performance of the aircraft. To alleviate this situation, the extendible leading edge concept of the present invention is proposed. This concept applies equally well for aircraft which require leading edge flap deployment (e.g. the F-18) and for those which do not require leading edge flaps (e.g. the F-15). The prior art abounds with examples of aircraft control systems. A number of patents disclosing typical inventions pertinent to the present invention will now be presented.
U.S. Pat. No. 5,344,103 to Fitzgibbon et al. discloses an actuating system for aircraft wing slat and flap panels. The actuating system provides an actuation mechanism for wing slats and flaps designed to avoid unexpected high stresses due to wing flexibility and, in particular, addresses the actuation mechanism of wing slats and flaps which may result in misalignment and interference loads due to wing bending.
U.S. Pat. No. 5,082,207 to Tulinius discloses an active flexible wing aircraft control system to improve aileron reversal speed, flutter speed, and load alleviation. The invention of this patent is said to utilize modern active flight control technology in which a number of smaller conventional control surfaces are activated to redistribute the airload so that desired roll rate and flutter suppression can be achieved within the domain of flight envelop.
U.S. Pat. No. 4,796,192 to Lewis discloses a maneuver load alleviation system whose purpose is to minimize wing root bending moment when threshold load level is exceeded during maneuvers or gust encounters, and thereby increase fatigue life of wing root components. In this instance, achieved automatically by means of control systems, conventional flaps are deflected upward to reduce the local angle of attack, although such an operation is not intended to improve aileron effectiveness in roll maneuvers
U.S. Pat. No. 4,729,528 to Borzachillo discloses an aeroelastic control flap whose purpose is to improve lateral and directional control during high stress maneuvers. This patented invention uses existing leading edge flaps and trailing edge flaps in a differential manner such that air load increases oil one wing and decreases on the other wing resulting in somewhat increased rolling moment, but at the cost of losing total lift. This loss of lift results in substantial altitude loss which is an undesirable occurrence in a combat area.
U.S. Pat. No. 4,651,955 to Krafka discloses a device for automatically controllable unloading of aircraft wings whose purpose is to minimize wing root bending moment when threshold load level is exceeded during maneuvers or gust encounters, and thereby increase fatigue life of wing root components. This is primarily a mechanical device which operates the control surfaces but would have no influence on improved roll performance.
U.S. Pat. No. 4,455,004 to Whitaker, Sr. discloses a flight control technique for aircraft in which spanwise movable control surfaces are provided at each wing tip. Its purpose is to minimize wing root bending moment during gust encounters to thereby increase fatigue life of wing root components. A secondary function is to provide additional rolling moment. These control surfaces must be located ahead of the elastic axis of the wing box. Extended booms are required to mount these control surfaces such that the concept would not be practical on fighter aircraft which carry wing tip missiles. Furthermore, adverse wing twist cannot be avoided by this invention.
U.S. Pat. No. 4,360,176 to Brown discloses a wing leading edge slat whose purpose is to provide smoothly contoured variable wing camber. This invention is mainly intended to provide aerodynamically smooth variable wing camber to achieve improved lift to drag ratio during takeoff. This patented invention does not address any maneuverable performance improvements.
U.S. Pat. No. 3,904,152 to Hill discloses a variable area, variable camber wing for aircraft whose purpose is to provide increased lift for short takeoff and landing operations. A mechanism is provided to extend the chord up to 70% and also increase the camber so that lift is increased without increasing pitching moment.
U.S. Pat. No. 3,486,720 to Seglem et al. discloses a continuous slot forming leading edge slats for cranked wings whose purpose is to provide continuous slot near the cranked wing junction of a supersonic wing. This patented invention describes a mechanism to deploy a wing slot near the cranked wing junction for increased lift for takeoff and landing of a supersonic transport aircraft but does not discuss maneuver performance nor aeroelastic effectiveness of control surfaces.
U.S. Pat. No. 2,938,680 to Greene et al. discloses a multiple position airfoil slat whose purpose is to provide an optimum wing leading edge configuration. This invention is particularly concerned with an improved type of automatically actuated slat for achieving an optimum wing leading edge configuration for widely separated flight environment conditions such as landing at low speeds and takeoffs with relatively short runs under heavy loads.
U.S. Pat. No. 2,343,986 to Leutholt discloses airfoil control for the purpose of providing lift, speed control and de-icing of an airfoil. An hydraulically actuated mechanism was developed to vary chord and effective camber of an airfoil so that desired lift and other characteristics required to meet different flight conditions could be achieved. This patented invention uses elastic material to cover the extended portion of the flap which introduces flutter problems in high speed flight vehicles.
U.S. Patent No. 1,427,012 to Page discloses an aircraft wing whose purpose is to increase the effective camber of the structure. An auxiliary wing is mounted so as to be parallel to and spaced from the leading edge of a primary wing and a mechanism is provided for selectively moving the auxiliary wing toward or away from the primary wing.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.